Operating systems are extremely customised. It is therefore almost impossible to find two similar systems in different environments. However, there are three major classifications determined by the mode of operation.
1. Hard RTOS
Hard RTOS have to perform its job on accurate time strictly. If a system has to finish the job and produce the result within a 5 second then it must show the result exactly by 5th second not 4th or 6th. If it do so, called a failed system. Hard RTOS also known as immediate RTOS. Example: Flight control systems, automotive systems, robotics , Railway signaling etc.
Hard RTOS guarantees the critical task also will complete on time. A hard RTOS is strictly follows the deadline or limits of the task which is given. In case this deadline is missed, the results are dangerous. For example, an airbag has to deploy within the perfect time frame. Failure to deploy may cause death or extreme destruction. It means that the deadline was missed.
2. Soft RTOS
Soft RTOS can’t guarantees about response time. It may or may not be in a probabilistic manner. i.e. response time is important but not problematic to the operation of a system. Soft system is priority based system. It first gives the priority to the critical tasks over other task. The time constraint is soft means though few misses occurs it’s not produces serious harm. Systems where deadlines are important but which will still function correctly if deadlines are occasionally missed. Instead of failure of results, system will perform poorly so the jobs will complete late. Example: Banking system, multimedia, DVD players, mobile phones etc.
The effects of missing a deadline with a soft RTOS are not as dangerous as in the previous case. The processes can be reversed or corrections made without much effect on the product or process. An example is the creation of a database that is used for storage purposes.
3. Firm RTOS.
Some applications that are neither fall into hard RTOS nor soft RTOS are called as firm RTOS. It provides the occasional deadline but discard the jobs which are not performed within a time. There is no value for a response that occurs past a specific deadline. Failure to meet the timing requirements is undesirable. A single system may have both hard and soft real-time Subsystems. In reality many systems will have a cost function associated with missing each deadline. Another characteristic of this system is that it rarely run on dedicated real time hardware.